1. Field of the Invention
This invention relates to an improvement on an electro-magnetically driven shutter.
2. Description of the Prior Art
Electro-magnetically driven shutters are in general arranged to have shutter blades drive by an electro-magnetic driven source to travel for an exposure and to have the shutter blades driven by return springs back to their travel start position. As disclosed, for example, in the specification of U.S. Pat. No. 4,302,090, in the conventional shutters of this kind, a current is continuously supplied to the electro-magnetic drive source until completion of an exposure to have the shutter blades kept in their travel completed positions against the forces of the return springs. However, since the driving force of the electro-magnetic drive source is extremely weak, a large amount of current is consumed in keeping the shutter blades in the travel completed positions against the force of the return springs. Therefore, since an apparatus such as a camera is generally allowed to have only a battery of a small capacity, the battery is used up in a short period of time. This has presented a problem.
Further, in the case of the conventional shutter of this kind, the current is allowed to flow to a moving coil for leading shutter blades under a locked condition always for a longer period of time than a moving coil for trailing shutter blades. This causes a rise in the temperature of the coil. As a result, the resistance value of the coil which has a temperature dependency increases to lower the amount of the current flow.
Meanwhile, the electro-magnetic force to be used for driving the shutter is proportional to the product of the current flowing to the moving coil and the number of turns of the coil. Therefore, the travelling speed of the leading shutter blade gradually decreases. The precision of the shutter eventually degrades according as the number of times of shutter release increases. This tendency is salient particularly in the case of a motor driven shutter.
Further, in order to have the shutter blades driven to precisely travel with the weak driving force of the electro-magnetic drive source, the above-stated return spring must have a very weak spring force. This results in a weak shutter blade holding force in the shutter travel start position. Then, the weak holding force has sometimes allowed an external shock to accidentally cause an exposure by dislocating the shutter blades from their positions within an aperture. It has been another problem that, with some minor vibrations applied to the camera, if the shutter blades deviate from their travel start position even to a slight degree, the timing for their travel deviates from correct timing and thus degrades the precision of the shutter.
Since the return springs are weak, the time required for returning the shutter blades becomes longer and the shutter operation becomes unstable. Therefore, a film winding speed becomes insufficient in the event of a consecutive photographing operation which is to be performed by a motor drive arrangement. A leak of a light flux tends to occur during the process of a return of the shutter blades. A further problem with the prior art shutters includes degradation of shutter precision due to a shortened time interval between completion of the travel of the shutter blades and the start of a next travel of them.